Thousands of undergraduate students are being forced to sign good behaviour contracts with their universities and warned they could be expelled if they breach regulations, the Guardian has learned.

The contracts put the onus on students to attend lectures and tutorials, but have been condemned by the National Union of Students. The NUS claims the contracts are "one-sided", and do not spell out what standard of teaching students should expect to get for the £3,000-a-year top-up tuition fees they are being charged.

Oxford and Chester Universities have introduced the contracts for students this year and legal agreements are already in place at Bristol and Nottingham Trent. The NUS believes it is the start of a disturbing trend that could be adopted by other universities.

At Oxford, which already makes such demands of its postgraduates, students must sign a document saying any breach of regulations or codes of practice about their conduct, studies and residences "may lead to your expulsion from the university or other sanctions".

Chester University says new undergraduates have an obligation to "study diligently, and to attend promptly and participate appropriately at lectures, courses, classes, seminars, tutorials , work placements and other activities which form part of the programme as required ..." The document also says academic courses differ widely "and the university does not therefore specify any minimum amount of teaching which you will receive ..."

The executive of the National Union of Students is challenging what it sees as a piecemeal, university-by-university process. It will oppose any arrangements not agreed with students and is calling for a debate over the obligations on all sides in an era when most undergraduates in England must use loans to fund their fees.

Three years ago, a Wolverhampton University law student received £30,000 in an out-of-court deal after claiming the course had not lived up to the prospectus.

Wes Streeting, the NUS's vice president for education, said: "Vice chancellors and principals are very busy protecting themselves from the customers they have created in a system they have been fighting for. It is absurd."

Contracts so far contain get-out clauses allowing universities to change the way courses are taught because of cuts, the need for institutional "efficiency" or changes in educational practice, as long as the alterations are "reasonable".

Both the government and the universities insist such contracts are a matter for individual institutions. Some make the contract a condition of entry. Oxford has a separate document which it requires students to sign. Mr Streeting doubted whether some students knew what they were getting into. "You don't really think about such nitty-gritty details. You have just got your A-level results, you are excited about getting to university."

Bristol's student agreement says it is "intended to provide a framework through which the university and its students can work together". Nottingham Trent said "a primary objective of this written contract, which is legally binding, is to highlight the obligations of both the university and its students."

Chester said its document "is designed to protect the legitimate needs of the university's conscientious students, and to safeguard its resources from potentially vexatious claims".

An Oxford University spokesperson said there had been no difficulties with the contract for graduate students. "Nor are we aware of any difficulties in the case of the undergraduate contract which was sent out to students some weeks ago."